1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses suited to be used for projecting video images.
2. Description of the Related Art
Recently, either projectors for casting a single video image onto a screen, or projection systems in which a pair of video images are projected onto a screen in superimposed relation, are widely used. To display the video image, either a cathode ray tube or a liquid crystal display panel is employed. In the case of the liquid crystal display panel, an illuminating unit is provided.
For the projection lenses, what is capable of zooming is used as, for example, described in Japanese Laid-Open Patent Application No. Hei 5-27174.
Some of these zoom lenses have large distortional aberration in the zooming region of from the shortest focal length to a middle focal length. To improve the illumination of the projected image, two projectors can be used as shown by M and N in FIG. 28A. In this case, the two projected images on the screen S have to fit in each other. As distortion is present, however, the two projected images deviate from each other in the marginal zone as depicted in FIG. 28B, where the image quality is lowered exceedingly, thereby giving rise to a serious problem.
Meanwhile, FIG. 29 schematically shows a liquid crystal projector. Three liquid crystal panels 21, 22 and 23 for different primary colors are followed, toward a light source 27, by respective condenser lenses 24, 25 and 26 in juxtapositioned relation. Mirrors 28a to 28c distribute the illuminating light. Other mirrors 28d to 28f combine the images. A projection lens 29 is arranged to form an image of the light source 27 in the pupil position thereof. In this instance of FIG. 29, a parabolic reflector is positioned behind the light source 27. For another case, a condenser lens is further used in front of the light source 27.
To improve the aperture efficiency, a microlens array may be attached to the liquid crystal display panel as shown in FIG. 30. In this case, the angle of incidence of the illuminating light on the liquid crystal display panel deviates from the normal (the telecentric form collapses) as it goes to the margin. Thus, eclipse occurs as shown in FIG. 31. In the prior art, therefore, another problem arises in that the liquid crystal display panel becomes darker toward the margin of the image frame.